1. Field of the Invention
This invention relates to protective coating systems for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a beta-phase nickel aluminide overlay coating and to a process for modifying the grain structure of the coating to improve its oxidation resistance and promote the formation of a protective oxide scale on the coating that is substantially free of spinels.
2. Description of the Related Art
Components of the turbine, combustor and augmentor sections of gas turbine engines are often protected by a metallic environmental coating and/or a thermal barrier coating (TBC), the latter of which is typically a ceramic material bonded to the component surface with a metallic bond coat. Coating materials that have been widely employed as environmental coatings and bond coats include diffusion aluminide coatings and MCrAlX overlay coatings (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element). The aluminum content of these materials provides for the slow growth of a strong adherent and continuous aluminum oxide layer (alumina scale) at elevated temperatures, which protects the coating and its underlying substrate from oxidation and hot corrosion. When the coating serves as a bond coat for a TBC, the alumina scale also chemically bonds the ceramic topcoat to the bond coat.
Other types of environmental coatings and bond coats that have been proposed include beta-phase nickel aluminide (NiAl) overlay coatings. In contrast to the aforementioned MCrAlX overlay coatings, which are metallic solid solutions containing intermetallic phases, the NiAl beta phase is an intermetallic compound that exists for nickel-aluminum compositions containing about 30 to about 60 atomic percent aluminum. Notable examples of beta-phase NiAl coating materials are disclosed in commonly-assigned U.S. Pat. No. 5,975,852 to Nagaraj et al., U.S. Pat. No. 6,291,084 to Darolia et al., U.S. Pat. No. 6,153,313 to Rigney et al., and U.S. Pat. No. 6,255,001 to Darolia. These NiAl compositions, which preferably contain a reactive element (such as zirconium and/or hafnium) and/or other alloying constituents (such as chromium), have been shown to improve the adhesion of a ceramic TBC, thereby increasing the spallation resistance of the TBC.
In addition to modifying the chemistry of NiAI-base environmental coatings and bond coats, investigations have been undertaken to determine the effect that the surface finish of beta-phase NiAI overlay coatings has on their oxidation resistance and their ability to adhere a ceramic coating. Commonly-assigned U.S. Pat. No. 7,150,922 to Spitsberg et al. discloses a process by which the grain structure of a beta-phase NiAI overlay bond coat can be modified to improve the spallation resistance of a ceramic TBC deposited on the bond coat. The NiAI overlay coating is deposited by methods that produce a generally columnar grain structure, e.g., physical vapor deposition (PVD) techniques, with the result that open grain boundaries are typically present at the coating surface, with the grains physically separated thereby being referred to as leaders. For NiAI coatings of the type taught by the aforementioned U.S. Pat. Nos. 6,153,313, 6,255,001 and 6,291,084, leaders are characterized by both a physical separation and chemical differences, and has been associated with reduced oxidation and spallation resistance as a result of providing a pathway for oxidation. To reduce the presence of deleterious leaders, Spitsberg et al. recrystallize the NiAI coating during or after deposition, resulting in the formation of new surface grain boundaries. Recrystallization is achieved by depositing the coating on a substrate maintained at a sufficiently high temperature so that recrystallization occurs during deposition, or by cold or warm working the coating surface and then heat treating the coating at a temperature sufficient to cause recrystallization.
A particular technique disclosed by Spitsberg et al. for cold or warm working the coating surface is shot peening. In practice, stainless steel shot peening media has been used. However, NiAl overlay coatings peened in this manner have been found to form an iron-containing spinel scale that can promote the spallation of a ceramic coating deposited on the coatings.